Numerous kinds of zoonotic helminths are found in cysts, abscesses, swellings, infarcts, and tumor-like growths. When an object found in histopathological sections is believed to be a part of a worm, or is recognized as such, there is an urge to make an exact identification of it so that its presence in the tissues can be explained and its significance in relation to disease can be understood. On present knowledge, relatively few kinds of worms can be identified in tissue sections even at a broad generic level. Because most identifications are inexact, and often incorrect, neither the range or the frequency of occult helminthic infection in man is well documented. However, detailed descriptions of features seen in sections have been made for a sufficient number of parasites to demonstrate the feasibility of making accurate, definitive identifications based on representative microsections. The objectives in the proposed study are 1) to describe and identify a number of worms that have been found, and are in future found, in human tissues, and (2) to develop through a series of descriptive studies on selected representative types a system of differential diagnosis based on histological and cellular features of parasites seen in sections and, in special cases, on the character of the surrounding, non-inflammatory host tissue reactions induced by them.